Alkalinity

For the majority of applications, it is important to maintain a stable pH. Alkalinity is a measure of the ability of the water to resist changes in pH, this is also known as buffering.

Alkalinity is made up of calcium and magnesium carbonates, bicarbonates, chlorides and sulfates, however it is conventionally always quoted as mg/L CaCO3.

Alkalinity can be measured as the total or M-alkalinity which measures all the hydroxide, carbonate and bicarbonate ions. The P-alkalinity only measures the hydroxide and carbonate ions. The M- and P-alkalinity measurements together can be used to calculate the O-alkalinity which is a measure of the hydroxide ions.

  • Drinking water – Moderate alkalinity is desirable in most water supplies. Strong alkaline water has an objectionable “soda” taste. The EPA Secondary Drinking Water Regulations limit alkalinity only in terms of total dissolved solids (500 ppm) and to some extent by the limitation on pH.
  • Wastewater – An alkali such as caustic soda can be used to provide the alkalinity required to maintain effective biological activity and pH control. After complete nitrification, a residual alkalinity of 50 mg/L in the aeration tank is desirable. The process of nitrification uses up carbonates and bicarbonates and increases the acidity of the water. As the bacteria are sensitive to drops in pH, alkalinity is important in these applications.
  • Aquaculture – Maintenance of total alkalinity levels protects against rapid pH changes. To protect aquatic life, the alkalinity should be at least 20 mg/L, although this will depend on the fish present.
  • Pool and spa – Alkalinity will prevent sudden pH changes when bodily matter is released into the water. Urine and sweat are acidic and can lead to pH changes if not correctly buffered. Above 120 mg/L CaCO3, the pool water can become cloudy.
  • Soil (irrigation) – Water may be unsuitable for use in irrigation if the alkalinity level in the water is higher than the natural level of alkalinity in the soil.

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